1. Field of the Invention
This invention relates to an image heating apparatus of the induction heating type for heating an image formed on a recording material.
2. Related Background Art
Various image heating apparatuses using the principle of electromagnetic induction have in recent years been proposed as an image heating apparatus like the fixing device of an image forming apparatus such as a printer or a copier.
An apparatus of this type is one which uses an electromagnetic induction heat generating body as a heating body and in which a magnetic field is made to act on this electromagnetic induction heat generating body by magnetic field generating means and heat is imparted to a material to be heated by Joule heat generation based on an eddy current produced in the electromagnetic induction heat generating body to thereby heat and fix an unfixed toner image on the surface of the material to be heated.
Japanese Patent Publication No. 5-9027 discloses an apparatus of the heating roller type in which a metallic fixing roller of a ferromagnetic material is electromagnetically induction heated., and this apparatus can make a heat generating position near to a pressure contact nip portion, and achieves a fixing process higher in efficiency than an apparatus of the heating roller type using a halogen lamp as a heat source. However, the heat capacity of the metallic fixing roller is great, and this has led to the problem that to make the temperature of the pressure contact nip portion rise by limited electric power, great electric power is required.
Japanese Patent Application Laid-Open No. 4-166966 discloses a fixing apparatus of the electromagnetic induction heating type using a film-like metallic fixing roller reduced in heat capacity. However, in the film-like metallic fixing roller reduced in heat capacity, the heat transfer in the lengthwise direction thereof (the longitudinal direction of the pressure contact nip portion) is hampered and therefore, there has arisen the problem that when paper of a small size is fed, an excessive temperature rise in a non-feeding portion.(non-feeding portion temperature rise) occurs to thereby reduce the life of the film or a pressure roller.
Actually the outer peripheral surface of the metallic fixing roller is heated until the temperature thereof becomes a temperature (e.g. 150xc2x0 C. to 200xc2x0 C.) suited for fixing, and the temperature of the interior of the metallic fixing roller also rises to the same temperature as that of the outer peripheral surface of the metallic fixing roller. Further, the heat generation by copper loss occurring due to an electric current flowing to a coil itself and the heat generation by the iron loss of a core are added, and the interior of the metallic fixing roller more excessively rises in temperature than the outer peripheral surface of the metallic fixing roller. Furthermore, in the non-feeding region, the material to be heated which takes the amount of heat of the outer peripheral surface of the metallic fixing roller does not pass and therefore, the outer peripheral surface of the metallic fixing roller in the non-feeding region rises to a temperature above the temperature suited for fixing, and the temperature of an excitation coil therein further rises and partly assumes an excessively heat rising state. In the narrow space in the fixing roller, sufficient radiation cannot take place and thus, a highly heat-resistant number is used as the coil itself, and this becomes a factor of an increased cost.
As a countermeasure for such excessive temperature rise of the non-feeding region, it is described in Japanese Patent Application Laid-Open No. 9-171889 that a magnetic flux shield plate rotatable in conformity with the size of a recording material is provided in cylindrical heat generating film.
Also, in U.S. Pat. No. 6,246,843, there is described a construction in which discretely from an excitation coil, a second coil for decreasing a magnetic flux is provided and a circuit having a second coil is opened or closed in conformity with the size of a recording material.
The present invention provides an image heating apparatus of the induction heating type which can change a heat generating area by a construction simpler than these propositions and in accordance with recording materials of various sizes.
The present invention has been made in view of the above-noted problem and an object thereof is to provide an image heating apparatus which can effect heat generation conforming to the size of a recording material.
Another object of the present invention is to provide an image heating apparatus which can effect heat generation conforming to the size of a recording material by a simple construction.
Still another object of. the present invention is to provide an image heating apparatus comprising:
a heating member;
an excitation coil for generating a magnetic flux to induce an eddy current in the heating member; and
a magnetic flux shield member coupled to the heating member for making the magnetic flux small;
wherein the magnetic flux shield width of the magnetic flux shield member in the longitudinal direction of the heating member differs in a direction intersecting with the longitudinal direction, and the magnetic flux shield member is movable in the direction intersecting with the longitudinal direction.